At lease six published studies show that bright light treatments reduce depressive symptoms both among patients with non-seasonal major depressive disorders and among patients with winter depression. The extent of benefit which can be gained and the optimal timing of treatment for different kinds of patients remains to be established, together with the mechanisms of action. This proposal will complete an on-going study to determine a) the benefit achieved by 7-day bright light treatment of patients with non-seasonal major depressive disorders, b) the predictors of favorable response, and c) effects of light treatment on melatonin excretion. In the on-going project and renewal, 72 psychotropic-drug-free patients with major depressive disorders will be studied in our Mental Health Clinical Research Center. After two days of baseline, half will receive 7 days of treatment with 1500-2500 lux fluorescent light and half will be treated with a control dim red light. Hamilton and Beck depression ratings, circadian mood self-ratings, Stanford Sleepiness Scale, 24-hour wrist activity measurements, EEG sleep recordings, and nocturnal urine samples for melatonin will be collected continuously for 11 days including 2 days of follow-up. As soon after completion of this protocol as possible, melatonin sensitivity to light suppression tests will be conducted on both the patients and age-matched healthy controls. For these tests, blood will be sampled one evening every half hour from 2030-0200, while subjects will be exposed to 500 lux light from 0100 to 0130. Analyses will examine predictors of mood responses and effects of light on melatonin.